Pitched roofs are the most common types of roofs. The pitch of the roof is a measurement of how steep the roof is which determines how quickly the water will run off of the roof. Roof pitches are typically measured by an equation called the "rise over the run" and is figured in the number of feet that the roof rises in a 12 foot horizontal run. A "12/12 pitched roof" means that the roof rises 12 feet in a horizontal run of 12 feet and results in a 45 degree angle. A 12/12 pitch is regarded as quite steep because it is impossible to stand on without some kind of support. A 3/12 pitch roof is regarded as quite shallow, and is the minimum pitch at which most roofing products will successfully shed water. Most pitched roofs are between 4/12 and 8/12 in pitch. At 1/12 pitch or less, a roof is considered to be "flat".
The most common type of roofing material for pitched roofs is composition shingles. However, there are many other types of roofing available including wood shingles and shakes, metal, slate, tile and other more exotic materials.
Flat roofs are roofs with zero pitch or with pitches so low that the water runs off too slowly for overlapped type of roofing materials to successfully shed water. Because the traditional types of roofing materials used on pitched roofs are not effective, different types of roofing materials have been developed to deal with this problem. For many years, that was limited to tar and gravel and flat seamed soldered metal. However, in the last twenty years or so, many new types of flat roof systems have been developed, including plastic, rubber and modified bitumen roof materials. The most common of these in use today on residential homes is modified bitumen. This material is made by combining asphalt (bitumen) with various chemicals whose purpose is to prolong the viable lifetime of the asphalt product. These chemicals fall into two basic categories, the first of which is elastifiers, whose purpose is to keep the asphalt flexible, and the second, UV blockers, whose purpose is to stop ultraviolet light from degrading the product. In combination, these chemicals greatly extend the lifetime of the asphalt. This material comes in large rolls which are then melted together to form one uniform sheet of material across the flat roof. Although tar and gravel is still used to some degree, it is fast disappearing from use because the modified bitumen is both cheaper and more effective.
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